Bug 623142 – Ensure ::new-connection runs before processing D-Bus messages

Without this guarantee, peer-to-peer connections are not very
useful. However, with this guarantee it's possible to export objects
in a handler for the GDBusServer::new-connection signal.

There are two caveats with this patch

 - it won't work on message bus connections
 - we don't queue up messages to be written

that can be addresses later if needed.

https://bugzilla.gnome.org/show_bug.cgi?id=623142



Signed-off-by: David Zeuthen <davidz@redhat.com>